1. Field of the Invention
This invention relates to polymeric compositions comprising fluoropolymers.
2. Introduction to the Invention
Fluoropolymers are well known and are widely used in a variety of applications due to their generally good physical and electrical properties and ability to withstand relatively high temperatures. Depending on their chemical composition, fluoropolymers can be thermoplastic, elastomeric, or thermoplastic elastomers. Commercially available fluoropolymers include polytetrafluoroethylene (PTFE), tetrafluoroethylene/hexafluoropropylene copolymer (FEP), perfluoroalkoxy resin (PFA), polychlorotrifluoroethylene (PCTFE), ethylene/tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVF.sub.2), and polyvinylfluoride (PVF). Typically these materials exhibit high melting temperatures, low glass transition temperatures, good solvent resistance, good insulation properties, low coefficient of friction, low flammability, low gas permeability, and good physical stability. Many of these fluoropolymers, however, are subject to stress-cracking, particularly if they are thermoplastic and if the conditions of use require exposure to elevated temperature. Those fluoropolymers such as PFA which have good stress-crack resistance tend to be expensive but usually have more than adequate physical properties, e.g. flexibility and creep resistance, to meet product requirements.
For some applications, it is desirable to blend two or more fluoropolymers in order to achieve advantages in terms of processability, temperature stability, or cost over the properties of the individual polymer components. For example, U.S. Pat. Nos. 4,624,990 and 4,859,836 (Lunk et al, issued Nov. 25, 1986 and Aug. 22, 1989, respectively) describe a melt-shapeable composition in which a first fluoropolymer of relatively low crystallinity, e.g. a copolymer of tetrafluoroethylene and a perfluorinated comonomer, and a second fluoropolymer of relatively high crystallinity which is not melt-shapeable in the absence of other polymers, e.g. irradiated polytetrafluoroethylene, are mixed to produce a highly crystalline material that can be melt-shaped, has a high melting point, and good tensile and elongation properties.
U.S. Pat. No. 5,057,345 (Barrett, issued Oct. 15, 1991) discloses a composition in which a fluorinated ethylene-propylene copolymer is mixed with a block copolymeric fluoroelastomer to produce a material with high tensile strength, low modulus, increased flexibility, and improved stress-crack resistance.
U.S. Pat. No. 5,041,500 (Ishiwari et al, issued Aug. 20, 1991) discloses a heterogeneous melt-processable blend of FEP and PFA suitable for making articles in which dimensional control is maintained. The disclosure of each of the patents referred to above is incorporated herein by reference.